The present invention is directed to a method for identifying a measured quantity proportional to a length difference between an optical element, which may be a fiber or a bundle of fibers, and an optical lead envelope, which receives the optical element as the optical lead passes through at least one cooling path.
In order to obtain a certain length allocation between optical elements, such as fibers or bundles of fibers, and a hose-shaped outside envelope, which is applied or formed by an extrusion, a manufacturing or processline is known wherein a certain optical fiber length is setby an adequate coupling of the optical fiber to an outside envelope. The coupling between an optical fiber and the envelope is thereby always achieved by looping around at least one large, easy running storage wheel that is situated at a prescribed temperature. A subsequent cooling effects the length change in the envelope wherewith a length allocation of the optical fiber or element to the envelope is achieved. An example of such a device is shown in U.S. Pat. No. 4,728,470, which is incorporated by reference thereto.
The actual length difference between the optical fiber and envelope thereby obtained cannot be identified with adequate precision with the measuring equipment employed up to now. German Published Application No. 34 25 649 discloses a measuring method wherein the length of the optical fiber in the envelope is acquired in that the draw-in rate of the optical fiber preceding the extruder and the speed of the optical lead following the cooling path are identified. Given such a comparative speed measurement between the optical fiber and the envelope, a precision on the order of magnitude of a few 10.sup.-3 is at most achieved in all length measuring methods known up to now. Since the length difference between the optical fiber and the envelope to be identified lies in the range of between 0 and 5.times.10.sup.-3, a systematic deviation between desired and achieved fiber lengths in the loose envelope cannot be identified with adequately great precision.